Ed Wrenbeck, former lead developer of the Siri iPhone app (before it was purchased by Apple):

For Siri to be really effective, it has to learn a great deal
about the user. If it knows where you work and where you live and
what kind of places you like to go, it can really start to tailor
itself as it becomes an expert on you. This requires a great deal
of trust in the institution collecting this data. Siri didn’t
have this, but Apple has earned a very high level of trust from
its customers.